The activity of antibody-drug conjugates (ADCs) on cancer cells can be affected by a multitude of factors, such as binding affinity, rate of internalization, subcellular trafficking, and efficient drug release within the target cell population. Consequently, the properties of an ideal antibody for drug delivery are not necessarily the same as those for a therapeutic unconjugated antibody. Furthermore, indirect assays involving the use of secondary antibodies to screen for optimal ADCs can be misleading, since crosslinking on the cell surface can lead to altered downstream events, and the affinity of the secondary antibody constrains the dynamic range of the assay. When seeking candidate antibodies directed against a novel antigen for ADC therapy, it is therefore most desirable to screen a large antibody panel in the form of ADCs and evaluate their cytotoxic activities, since these results provide a direct measurement of parameters that can affect cytotoxic activity. However, when dealing with microgram quantities of a large number of antibodies as is typical of an antibody discovery campaign, the yields from conventional conjugation methodologies are limiting. A need exists for improved methods of screening antibodies for use as ADCs. This present invention addresses this and other needs.